Background

Notch signaling is activated upon engagement of the Notch receptor with its ligands, the Delta, Serrate, Lag2 (DSL) single-pass type I membrane proteins. DSL proteins contain multiple EGF-like repeats and a DSL domain that is required for binding to Notch (1,2). Five DSL proteins have been identified in mammals: Jagged1, Jagged2, Delta-like (DLL) 1, 3, and 4 (3). Ligand binding to the Notch receptor results in two sequential proteolytic cleavages of the receptor by the ADAM protease and the γ-secretase complex. The intracellular domain of Notch is released and then translocates to the nucleus where it activates transcription. Notch ligands may also be processed in a similiar manner, suggesting bi-directional signaling through receptor-ligand interactions (4-6).

TNF-α converting enzyme (TACE), also known as ADAM17, is a transmembrane metalloprotease that plays a key role in the cleavage of a number cell surface molecules in a process known as “shedding". TACE is abundantly expressed in many adult tissues, but in fetal development, expression is differentially regulated (7). TACE activates Notch in a ligand-independent manner and has been shown to play a role in the development of the Drosophila nervous system (8).

Recombining Binding Protein, SUppressor of Hairless (RBPSUH), also termed RBP-J or CSL, is the DNA-binding component of the transcription complex regulated by canonical Notch signaling. In the absence of Notch activation, RBPSUH suppresses target gene expression through interactions with a co-repressor complex containing histone deacetylase. Upon activation of Notch receptors, the Notch intracellular domain (NICD) translocates to the nucleus and binds to RBPSUH. This displaces the co-repressor complex and replaces it with a transcription activation complex that includes Mastermind-like (MAML) proteins and histone acetylase p300, leading to transcriptional activation of Notch target genes (9-11).

Numb contains an amino-terminal phosphotyrosine-binding (PTB) domain and carboxy-terminal endocytic binding motifs for α-adaptin and EH (Eps15 homology) domain-containing proteins, indicating a role in endocytosis (12,13). There are four mammalian Numb splicing isoforms that are differentially expressed and may have distinct functions (14-16). Numb acts as a negative regulator of Notch signaling by promoting ubiquitination and degradation of Notch (17). The protein is asymmetrically segregated into one daughter cell during cell division, producing two daughter cells with different responses to Notch signaling and different cell fates (18,19).